<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form method="POST" action={{ url_for(".login") }}>
    <input type="text" name="user_name" placeholder="user_name">
    <button type="submit">提交</button>
</form>
</body>
</html>